Electrochemical production of periodate oxypolysaccharides



July 19, 1955 c. L.. MEHLTRETTER ELECTROCHEMICAL PRODUCTION OF PERIODATEOXYPOLYSACCHARIDES Filed July 25, 1954 l f :mss

ATTORNEYS Z,7l3,553 Patented July i9, 1.955

ELECTRGCHEMCAL PR-DUC'llQN F Pilki@- DTE @XYPGLYSACCHAEHDES Charles l2.lvlehltretter, Peoria, lll., assigner to the United States of America asrepresented by the Secretary of Agriculture Application .lilly 23, i954,Serial No. 445,439 3 Claims. (Cl. 23d-79) (Granted under Title 35, U. S.@ode (1952), sec. 266) A non-exclusive, irrevocable, royalty-freelicense in the invention herein described, for all governmentalpurposes, throughout the world, with the power to grant sublicenses forsuch purposes, is hereby granted to the Government ot' the United Statesof America.

This invention relates to a method for producing oxypolysaccharides suchas oxystarch, oxycellulose and the like, and is a modification of theelectrolytic process described and claimed in the Dvonch and MehltretterPatent No. 2,648,629. it relates to a method involving the use ot' analkaline catholyte, and attendant technological advantages over theprior use of an acidic catholyte.

According to the invention disclosed and claimed in the patentpreviously mentioned, a polysaccharide material such as starch istreated in an electrolytic cell containing small amounts of periodicacid. An electric current is passed through the system to regenerateperiodic acid as it is consumed by reaction with the starch. Thus, theperiodic acid, upon reaction with the starch, oxidizes it and is itselftransformed to iodic acid. This iodic acid is then regenerated toperiodic acid, and the reaction cycle recurs.

ln the prior method a compartmented cell is employed using a leaddioxide anode in an anolyte compartment separated by a porous partitionfrom the cathode in an acidic catholyte. At the end of each reactionperiod the anolyte is separated and the oxidiezd material recoveredtherefrom. As disclosed in the patent, it has the technologicaladvantage of requiring only very small amounts of expensive periodicacid.

In the prior invention usino a 2 percent sulfuric acid catholyte,sulfate ions enter the anolyte by electrolysis to produce sulfuric acidwhich builds up on recycling or reuse of the anolyte, and eventually adeleterious excess is created which must be removed. This istechnologically diflicult because it must be done without loss of iodioor periodic acid. The common precipitants for sulfuric acid also tend toprecipitate iodic and periodic acids. Also the accumulated high acidityof the anolyte is a source of possible damage by hydrolysis to thepolysaccharide being treated as well as the oxidized polysaccharide.

The present invention is an improvement over this prior patentedprocess, the major feature being the use of an alkaline catholyte. lnthe present invention the anolyte contains the iodate (or periodate) insalt form or as the free acid, as for example sodium iodate or iodioacid. lt also contains an electrolyte salt such as sodium sulfate forthe purpose of achieving good conductivity of the solution and thussaving electric current. In the system of the present invention thepassage of electric current causes sodium ions to transfer to thecatholyte and hence build up its alkalinity. The negative ions of theelectrolyte additive transfer to the anode to make the anolyte acid.This acidifying effect of the anolyte is supplemented by the transfer ofiodate and periodate ions taking part in the chemical reaction. Thisacidity which builds up in the anolyte, however, is considerably lessthan that slurried with anolyte recycled from produced with the sulfuricacid catholyte and is a constant amount for a denite period ofoxidation. lt may be partially or completely neutralized during theoxidation process or after removal of the oxidation product at thecompletion of the oxidation. A suitable source of alkali for thispurpose is that which is formed in the catholyte chamber or sodiumbicarbonate formed by reaction of the alkali from the catholyte withcarbon dioxide.

Neutralization of the acidity which is continuously generated in theanolyte occurs during the oxidation by passage of alkali into theanolyte from the catholyte chamber, through the porous partitionseparating the two. ln order for this passage of alkali to beappreciable, it is desirable that a hydrostatic head of pressure bemaintained so that flow occurs from the catholyte to the anolyte, i. e.the surface of the catholyte is hence kept higher than the surface ofthe anolyte. This feature of operation has an additional major purposein preventing migration of iodate and periodate ion to the catholytechamber. rthis latter function is important in preventing loss of theexpensive iodate reagent.

T he rate of passage of alkali static pressure and also by the porosityof the partition. By using a partition of proper porosity andmaintaining proper hydrostatic pressure from the catholyte to theanolyte it is feasible to induce sufficient transfer of, alkali tomaintain the pli of the anolyte at the desired value. I prefer, however,to supplement this by recirculation of excess alkali as shown in theilow sheet drawing by recycling to the anolyte a portion of thewithdrawn and diluted catholyte. This oilers positive control over thehydrogen ion concentration in the anolyte. The latter should be keptbelow pH S, preferably pH 2 to pH 5, in order to suppress hydrolysis ofthe polysaccharide.

The novel system of this invention, when practiced as disclosed herein,is inherently self-sustaining except for the polysaccharide beingoxidized. Since the quantity of negative electrolyte ions, for example,sulfate and iodate ions, present in the anolyte will always be the sameduring the operation of the cell, it follows that the total quantity ofelectrolyte salt and sodium iodate will remain constant in the recycledliquors except for small additions of these salts to make up for slightlosses during processing. Hence, no other chemicals are required by thesystem, and no build-up of acid in the anolyte can occur because ofrecycling.

The invention has a further technological advantage, in that high yieldsof oxidized polysaccharide are obtained because of low hydrolysis loss.Moreover, the recycled anolyte contains fewer impurities becauseaddition of foreign substances to remove accumulated acid as in theprior patent process is not required.

Although it is preferred to carry out the process as discussed abovemaintaining the pH of the anolyte within the range oi pH 2 to pH 8, ithas nevertheless been found that the process can be carried out withoutneutralizing the acidity of the anolyte. lt has been found for examplethat the process can be carried out at a hydrogen ion concentrationconsiderably below pH 2, if desired.

The anolyte of sodium iodate and sodium sulfate solution may also beelectrolyzed for several hours to build up the periodate concentrationand the solution neutralized with alkali preliminary to the addition ofthe polysaccharide to the cell. This reduces the time ot exposure of thepolysaccharide to the acid anolyte and consequently reduces any lossesdue to hydrolysis.

The accompanying drawing illustrates an embodiment of the process. ltshows the liow of materials in a conipiete process wherein starch isoxidized electrochemically to oxystarch. Starch is fed into mixer lwhere it is the process. From is governed by the hydromixer 1 the starchis fed to the oxidizing cell 2. The cell is provided with anode 3,immersed in anolyte 3a and cathode 4 situated inside porous compartment5 containing catholyte 4a.

While in the anolyte part of the cell, the starch is oxidzed chemicallyby contact with periodate ions formed by oxidation of iodate ions at thelead dioxide electrodes (anodes) and after oxidation, is removed fromthe bottom of the cell while suspended in the anolyte and pumped throughline 6 to settling tank 7. From the settling tank the settled oxidizedstarch is removed as a pulp where it may be recovered by any suitablesystem of recovery. n the drawing it is shown passing to storage tank 3from which it is led to a filter installation 9 where it is filtered andwashed countercurrently to displace the associated anolyte. Therecovered anolyte is then sent to a concentrator 10 for evaporating theWash water used and re-establishing the required concentration ofanolyte for recycling to mixer l through line il. The anolyte from thesettling tank 7 is decanted and used for mixing with the feed starch aspreviously noted.

The steel cathode 4 is situated in porous compartment S as previouslynoted, and in this compartment the sodium ions which migrate underinuence of the electric current are converted to sodium hydroxide. Someof this sodium hydroxide passes to the anolyte through the porouspartition and partially neutralizes the acids formed during theelectrolysis. Further neutralization to the desired pH is accomplishedby the direct addition of alkali to the anolyte, either before or afterremoval of the oxypolysaccharide by filtration. The alkali used for thispurpose may be taken from the excess alkali removed from the catholyteas shown in the iow chart. The alkalinity in the catholyte may bereduced by withdrawing catholyte to vessel 12 where it is adjusted tothe required concentration by dilution with water for eventual recyclingthrough line .1.3 to neutralize anolyte acidity.

The process may be carried out either batch wise or by continuous orsemi-continuous operation. As shown in the drawing the recovery of theproduct may be accomplished by any suitable recovery system, which mayinvolve the use of a drum type filter or centrifuge for the recovery ofsubstantially pure oxystarch.

The conditions of electrolysis used in this invention are substantiallythose required in the previously patented process. For example, theanolyte is preferably well stirred to maintain homogeneity and atemperature mainn tained within the range of 5 C. to 50 C. The amount ofcurrent required may vary over a rather wide range. It has been foundthat a current density of from 0.5 to 5 amperes per square decimeter ofanode surface gives good results.

The present invention affords a wide range of degree of oxidation ofpolysaccharides, t'he degree substantially ranging from 0 to 100percent. One factor which controls the degree of oxidation is theconcentration of iodic acid and sodium iodate. When relatively lowconcentrations of sodium iodate are employed, longer times are requiredto obtain a given degree of oxidation. However, the time may beshortened by increasing the current. Furthermore, if relatively largeconcentrations of sodium sulfate are employed, the oxidation can golonger before it is necessary to neutralize to maintain effectiveconductivity of the cell.

The following examples illustrate the invention. ln each, anelectrolytic cell was employed consisting of two cathode compartments ofCoors No. 730 size l porous clay cups. Other acid and alkali resistantmaterials, such as asbestos, may also be used to separate the anode andcathode compartments. The cathodes were of steel of about l0 sq. cm.surface area, immersed in the sodium hydroxide solution placed insidethe clay compartments. The concentration of alkali in the catholyte wasmaintained at 2-5 percent throughout the oxidation by dilution withWater. The catholyte surface was higher than the anolyte surface toprevent loss of anions by migration through the diaphragms into thecatholyte.

The anode was a lead dioxide battery plate or lead coated with leaddioxide by electrolysis for 8 hours in 6 N sulfuric acid at 3 amperesand was from 60-80 sq. cm. in effective surface area.

Example 1 The two cathode compartments of a three compartment cell wereeach charged with 2O ml. of 5 percent sodium hydroxide solution. To theanode compartment was added 300 ml. of 2 percent sodium iodate-8 percentsodium sulfate solution and 50 grams of defatted corn starch on a drybasis.

A few drops of octyl alcohol were added to control foaming. After i9hours of electrolysis at 2 amp. and 5 volts at a temperature of 20 C.the anolyte was filtered and washed free of iodate with water. The whiteproduct was then stirred in acetone and filtered. After equilibration at65 percent r. h. and 20 C. for several days the product weighed 58.5 g.(Moisture 18.47 percent, yield percent.) lt analyzed 77 percentdialdehyde starch.

The filtrate was clear and colorless and after neutralization andconcentration to the appropriate volume could be recycled in theprocess.

Example 2 Using the same set-up as in Example l and an anolyteconsisting of 300 ml. of an aqueous solution of 3 percent sodium iodateand 8 percent sodium sulfate and 50 g. (0.3) defatted corn starch,electrolysis at 2 amp. was carried out for 24 hours at 20 C. Gradualneutralization of the anolyte with 40 percent sodium hydroxide wasperformed to maintain a pH of 2 to 5 in the anolyte. The oxystarch wasisolated as in Example l, and a product was obtained in a yield of 92percent which gave an analysis of 97 percent oxystarch on a dry basis.Recovery of iodate and periodate in the ltrate and Washings was 98percent. The catholyte contained no iodate or periodate.

Example 3 Using the same set-up as in the previous examples and ananolyte consisting of 300 ml. of a solution containing 26.5 g. iodicacid, 45 g. sodium sulfate and 162 g. (D. E.) defatted Corn starch,electrolysis was carried out at 2 amperes for 23 hours. rThe oxystarchwas isolated as in the previous examples. The product was obtained in :tyield of 98 percent, and analyzed 62 percent oxystarch on a dry basis.

I claim:

l. Method for the preparation of oxypolysaccharides comprisingsubjecting a polysaccharide material to the effect of substantiallycatalytic amounts of periodic acid in the presence of a cathode and alead dioxide anode at a temperature of about from 5 C. to 56 C., passingelectric current through the system, said periodic acid beingcontinuously transformed to iodic acid by chemical reaction with thepolysaccharide material and said iodic acid being continuouslyregenerated to periodic acid at the anode, and continuing said electriccurrent until substantial amounts of the polysaccharide are oxidized,said cathode being immersed in an alkaline catholyte confined in acatholyte compartment and said anode being immersed in an anolyte, andcounteracting the acidity which tends to build up in said anolyte byrecycling to said anolyte a portion of withdrawn and diluted catholyte.

2. The method of claim l in which the pH of the anolyte is maintainedwithin the range of 2 to 5.

3. The method of claim l in which the polysaccharide is starch.

References Cited in the file of this patent UNITED STATES PATENTS2,648,629 Dvonch et al. Aug. ll, 1953

1. METHOD FOR THE PREPARATION OF OXYPOLYSACCHARIDES COMPRISINGSUBJECTING A POLYACCHARIDE MATERIAL TO THE EFFECT OF SUBSTANTIALLYCATALYTIC AMOUNTS OF PERIODIC ACID IN THE PRESENCE OF A CATHODE AND ALEAD DIOXIDE ANODE AT A TEMPERATURE OF ABOUT FROM 5* C. TO 50* C.,PASSING ELECTRIC CURRENT THROUGH THE SYSTEM, SAID PERIODIC ACID BEINGCONTINUOULSY TRANSFORMED TO IODIC ACID BY CHEMICAL REACTION WITH THEPOLYSACHARIDE MATERIAL AND SAID IODIC ACID BEING CONTINUOUSLYREGENERATED TO PERIODIC ACID AT THE ANODE, AND CONTINUING SAID ELECTRICCURRENT UNTIL SUBSTANTIAL AMOUNTS OF THE POLYSACCHARIDE ARE OXIDIZED,SAID CATHODE BEING IMMERSED IN AN ALKALINE CATHOLYTE CONFINED IN ACATHOLYTE COMPARTMENT AND SAID ANODE BEING IMMERSED IN AN ANOLYTE, ANDCOUNTERACTING THE ACIDITY WHICH TENDS TO BUILD UP IN SAID ANOLYTE BYRECYCLING TO SAID ANOLYTE A PORTION OF WITHDRAWING AND DILUTE CATHOLYTE.